The invention relates to a gas discharge lamp, particularly a fluorescent lamp, comprising a gastight, transparent lamp envelope including a filling gas containing mercury and a phosphor coating, and comprising two electrodes.
The generation of light in gas discharge lamps is based on a process wherein charge carriers, in particular electrons, but also ions, are accelerated so strongly by an electric field between the electrodes of the lamps that collisions with the gas atoms or molecules in the filling gas of the lamps cause these gas atoms or molecules to be excited or ionized. When the atoms return to the ground state, and also in the case of the recombination of electrons and ions, a more or less substantial part of the potential energy is converted to radiation.
Fluorescent lamps are low-pressure gas discharge lamps comprising a gas filling containing mercury and a phosphor coating on the lamp vessel. The spectrum of the radiation emitted by mercury comprises, in addition to a proportion of visible light, a very large proportion of UV radiation which is invisible to the human eye. This UV radiation impinges on the phosphor or the phosphor mixture on the inner surface of the lamp envelope. The phosphors absorb the radiation and emit radiation of a longer wavelength, i.e. visible light, instead. The chemical composition of the phosphor coating determines the spectrum of the visible light generated and hence the color temperature thereof. By a suitable choice of the phosphors, it becomes possible to give the light of the fluorescent lamp any desired color. The visible color depends upon the intensity ratio in the wavelength spectrum of the radiation generated; the brightness is determined by the overall intensity.
To generate white light, use is made of special phosphors or phosphor mixtures whose radiation is particularly intense in the red, green and blue spectral ranges, resulting in the light being perceived as xe2x80x9cwhitexe2x80x9d light. For example, conventional tri-phosphorus fluorescent lamps comprise the triphosphors BaMgAl10O17:EU(BAM) having an emission band at 450 nm, CeMgAl11O19:Tb(CAT) having an emission band at 545 nm and Y2O3:Eu(YOX) having an emission band at 612 nm. The emitted wavelengths of the three triphosphors are each near the angular points of the CIE standard chromaticity diagram and sum up to an emission spectrum whose color point comes close to that of a black body. While the color rendering index CRI of a black body is standardized on 100 at each temperature, the color rendering index of tri-phosphorus fluorescent lamps ranges approximately from 80 to 95.
Fluorescent lamps are manufactured in three color temperature ranges, namely daylight white with a color temperature in the range from 5,000 to 6,000 K, neutral white with a color temperature in the range from 4,000 to 4,300 K and soft-white with a color temperature in the range from 2,700 to 3,200 K.
However, fluorescent lamps of an even softer light color with a color temperature below 2700 K are also in demand, which demand has hitherto been met by incandescent lamps having said highly appreciated light color. Such a color temperature cannot be attained by means of a triphosphor mixture. To attain low color temperatures, a part of the visible blue Hg radiation at 400 to 450 nm must be removed from the wavelength spectrum. This can be achieved in known manner by means of a Cer-activated aluminate phosphor, a magnesium-germanate phosphor or yttrium-aluminium-granate (YAG) (Kirk-Othmer, Encyclopedia of Chemical Technology, 4th edition, New York, Interscience, vol. 15. p. 580). A fundamental drawback of these phosphors is their small absorptivity.
Therefore, it is an object of the invention to provide a gas discharge lamp, in particular a fluorescent lamp, comprising a gastight, transparent lamp envelope including a filling gas containing mercury and a phosphor coating, and comprising two electrodes, which gas discharge lamp emits soft-tone light, has a high color rendering index and can be manufactured in a cost-effective manner.
In accordance with the invention, this object is achieved by a gas discharge lamp comprising a gastight, transparent lamp envelope including a filling gas containing mercury and a first phosphor coating, and comprising two electrodes, the first phosphor coating containing a perylene pigment. The perylene pigment in the phosphor coating acts as a color converter. It absorbs the mercury radiation in the blue and near UV range at 436 and 405 nm, and emits radiation in the longer wavelength range extending from green via yellow to red. The perylene pigments combine a very good absorptivity in the blue and near UV range with a high absorption coefficient xcex5=100,000 l/cmxc2x7mol and an emission-quantum yield greater than 90%. Unlike other organic color pigments, they are not decomposed by UV radiation.
Within the scope of the invention, it is preferred that the first phosphor coating additionally contains a triphosphor mixture.
It is particularly preferred that the gas discharge lamp comprises a second phosphor coating containing a triphosphor mixture.
It is more particularly preferred that the second phosphor coating comprises BaMgAl10O17:Eu, CeMgAl11O19:Tb and Y2O3:Eu. This gas discharge lamp enables a color temperature below 2700 K to be attained at a color rendering index above 90.
It may also be preferred that the first phosphor coating contains organic polymers. Perylene pigments can be readily dispersed in organic polymers, where they form a coating which also increases the breaking strength of glass.
It is particularly preferred that the first phosphor coating contains N,Nxe2x80x2-bis (2,6-diisopropyl phenyl) perylene-3,4,:9,10 tetracarboxylic acid diimide. In a layer thickness of 1 mm, this perylene pigment absorbs 99% of the Hg emission at 436 nm and 50% of the Hg emission at 405 nm.
If the first phosphor coating contains two perylene pigments, a double color conversion can be attained.
Within the scope of the present invention, it is preferred that the gas discharge lamp comprises a phosphor coating containing a triphosphor mixture.